In recent years, higher speed in the transmission speed of data is demanded with increase in CPU clock frequency. However, in the conventional electric wiring, crosstalk and electromagnetic radiation become significant with higher speed of the transmission speed, and thus there is a limit to increasing the speed of the transmission speed. A method of connecting the CPU and various types of application circuits with a light transmission path such as a light guide, and transmitting a data signal as an optical signal has been attempted.
The light guide has a double structure of a center core, which is called a core, and a capsule covering the center core, which is called a clad, where the index of refraction of the core is higher than that of the clad. The optical signal that entered the core is propagated by repeating total reflection inside the core.
The schematic configuration of the light transmission module arranged in the light transmission path is described below using the drawings. FIG. 21(a) is a perspective view showing an outer appearance of the light transmission module, and FIG. 21(b) is a perspective view showing an inner appearance of a foldable portable telephone incorporating the light transmission module. FIG. 22 is a block diagram of a portion applied with the light transmission module in the foldable portable telephone incorporating the light transmission module.
A light transmission module 100 is configured by a light transmission processing unit 2 with a light source drive circuit (light emission drive portion) and a light emitting portion (light emitting element; VCSEL (Vertical Cavity-Surface Emitting Laser)) connected to a master side board (main control board) 20 mounted with a CPU 29; a light reception processing unit 3 with a light receiving portion (light receiving element; PD (Photo-Diode) and a reception (amplifier) IC connected to a slave side board (application circuit board) 30 mounted with various types of applications such as an LCD (Liquid Crystal Display), an LCD driver 39 for drive controlling the LCD, and a camera module; and a light transmission path 4 for transmitting an optical signal by connecting the light transmission processing unit 2 and the light reception processing unit 3, such as an optical fiber or a light guide having high bendability.
The mechanism of light transmission in the light transmission module 100 will now be briefly described. First, the light emission drive portion drives the light emission of the light emitting portion based on an electrical signal input from a main control board 20, and the light emitting portion irradiates a light incident surface of the light transmission path 4 with light. The light applied to the light incident surface of the light transmission path 4 is introduced into the light transmission path 4, and exit from a light exit surface of the light transmission path 4. The light exit from the light exit surface of the light transmission path 4 is received by the light receiving portion and converted to an electrical signal. The converted electrical signal is amplified to a desired value by an amplifier, and input to the LCD driver 39 and the like of the application circuit board 30.
The use of such light transmission module enables high speed and large capacity data transmission from the main control board mounted in the portable telephone to the application circuit board. Thus, the light transmission module excels as a data transmission module.
The light transmission module has an advantage in that large capacity data can be transmitted at high speed, but has a drawback in that the power consumption is large compared to the electrical transmission module for transmitting small capacity data at low speed. Thus, in spreading the light transmission module in the future, it is important to reduce the power consumption in the light transmission module, in particular, in a light transmission processing unit and a light reception processing unit.
To reduce the power consumption of the light transmission processing unit and the light reception processing unit, start and stop of the light transmission processing unit and the light reception processing unit are controlled according to the presence of data (transmitting data) transmitted between the units. The configuration shown in FIG. 23 and the configuration of patent document 1 shown in FIG. 24 are illustrated by way of example.
FIG. 23 is a block diagram showing a schematic configuration of a portable telephone including a light transmission module showing a technique conventionally used in general for controlling start and stop of the light transmission processing unit and the light reception processing unit. As shown in the figure, in the conventional technique, start and stop of a light transmission processing unit (Tx) 2 and a light reception processing unit (Rx) 3 are individually controlled based on a control signal input from the CPU 29 and a CPU (not shown) on the data reception side. Specifically, the CPU 29 on the data transmission side detects the presence of an input signal (data signal) to the light transmission processing unit 2 and controls the start and stop of the light transmission processing unit 2, and the CPU on the data reception side detects the presence of an output signal (data signal) from the light reception processing unit 3 and controls the start and stop of the light reception processing unit 3.
FIG. 24 is a block diagram showing a schematic configuration of the light transmission module including a light reception circuit disclosed in patent document 1. As shown in the figure, such light transmission module includes a signal detection circuit 35 for determining whether or not the light receiving portion 31 received the optical signal, and a power supply controller 34 for controlling the power supply of an amplifier 32 and an I/F circuit 33 based on the detection result of the signal detection circuit 35. The power supply controller 34 stops the power supply of the amplifier 32 and the I/F circuit 33 if the light receiving portion 31 has not received the optical signal, and the power supply controller 34 starts the power supply of the amplifier 32 and the I/F circuit 33 if the light receiving portion 31 has received the optical signal. In the light transmission processing unit 2, the power supply in the I/F circuit 21, the light emission drive portion 22, and the light emitting portion 23 can be controlled by the control signal input to the power supply controller 25 from the CPU (not shown) arranged outside.
According to such conventional techniques, the power consumption of the light transmission module can be reduced since start and stop of the light transmission processing unit and the light reception processing unit can be controlled according to the presence of the input signal (data signal).    Patent document 1: Japanese Unexamined Patent Publication No. 2004-135188 (date of publication: Apr. 30, 2004).